Help me defend Cookie on Apple Support Community

I just had a Level 10 user, meaning they're supposed to be an expert, tell me flat out that third party cookie management is not necessary on a Mac. I feel they couldn't be more wrong, but other than point out that cookie is "automated privacy" as you description says, I didn't do a good job of defending Cookie.
The question came up because I'm noticing that the upper right corner of the Google window started acting erratically some time in the last 3 weeks- the user icon disappears so that I can't log out and the little matrix of squares representing the Google apps comes and goes. The Level 10 person is blaming Cookie for this problem with Google.
Can anyone help me defend Cookie? I'm still on macOS El Capitan and Safari 10.0.1 :dodgy:
Here is what Level 10 said:

Third party cookie management apps are not necessary on a Mac and may be causing a conflict with Google. Uninstall that app
Then go to your Safari menu bar, click Safari > Preferences then select the Privacy tab then click either Remove All Website Data or Manage Website Data. Wait for cookies to populate then click Remove All
Quit then relaunch Safari to try Gmail.